[Alt-Tab] Fix minor regression (arrows not hidden after change in selection).

Regression was caused by "Refactoring: use accessors to reduce the number of duplicated variables."
[Alt-Tab] Draw the arrows also in non-thumbnail mode, pointing up.
[Alt-Tab] Add a workspace-separator also for empty workspaces.
[Alt-Tab] Show a window thumbnail behind the icon (5). Only for some alt-tab styles.
[Alt-Tab] Somewhat less code for showing previews.
[Alt-Tab] Show a window thumbnail behind the icon (6). Try a 3/4-size icon.